Add Thymeleaf extension

[jlouvel]

Base on @ggodlewski contribution mentioned in #746 we should add this extension to version 2.2 RC1:
https://github.com/ggodlewski/org.restlet.ext.thymeleaf

Signed JCA is already available.

Additional pointers:
https://github.com/thymeleaf/thymeleaf
http://www.thymeleaf.org/

[thboileau]

Thymeleaf extension integrated.